What is vegan diet?

Vegans choose their diet for a variety of reasons. Many abstain from animal products because they find factory farming cruel and inhumane. Others believe that animal agriculture destroys the environment. And some simply view veganism as a healthy diet -- one inherently low in fat, calories, and cholesterol and high in fiber and vitamins.

Moral Reasons

Most vegetarians feel ethically unable to eat animals slaughtered for meat. Vegans also believe that factory-farmed egg-laying chickens and dairy cows lead unnecessarily miserable and brief lives. Egg-laying chickens raised in factory farms usually live in small cages with no room to stretch or walk. Although they can live more than a decade, most are slaughtered after two years.

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Nonvegans who dislike factory-farm methods often choose free-range animal husbandry as an alternative. The United States Department of Agriculture requires that free-range chickens have access to an outdoor area. They do not, however, specify how much time chickens spend outside of pens and cages [source: USDA­]. Because of this loophole, vegans who choose the diet for moral reasons believe free range is unreliably humane.

Vegans also object to treatment of dairy cows. Like chickens, dairy cows are killed when their productivity rates drop. Cows can live up to 25 years, but most dairy cows die after five. Male calves, which have no value to dairy farms, are quickly sold and slaughtered in veal farms.

Environmental Reasons

Some people become vegans for environmental reasons. Animal agriculture requires vast amounts of land and water to support livestock and grow feed. The land conversion needed for pastures and feed crops leads to deforestation, soil degradation and a decline in biodiversity. The water used by animal agriculture, mostly as irrigation for feed crops, accounts for 8 percent of global human water use [source: Livestock, Environment and Development Initiative].

Enormous crops of corn or grain also require pesticides and fertilizers. The nitrogen in fertilizers contributes to algal blooms and creates ­dead zones.

­Methane, a greenhouse gas 23 times more powerful than CO2­, makes up the majority of livestock emissions. Animal manure contaminates water and releases antibiotics that may increase the resistance of some bacteria.

Vegans also believe that their diet uses land more efficiently and responsibly than animal agriculture. If people ate only plant-based food, there would be no need for the swaths of pastures and feed crops that support livestock. Agriculture would produce food directly for human consumption and use less land. The proteins and vitamins from animal products do, however, help sustain much of the world's poor -- people who might not have the resources to create and monitor a total plant-based diet.